Effects of acute exposure to cold (10 degrees C for 60 min) on the concentrations of zinc and related proteins in red blood cells and in plasma were studied on seven male students. Lower concentrations of total zinc and of zinc derived from carbonic anhydrase I (CA-I) in red blood cells were found during cold exposure and 15 min after cold exposure, whereas no significant changes occurred in the concentration of zinc derived from carbonic anhydrase II (CA-II) or Cu2Zn2 superoxide dismutase (SOD1). The decrease in total zinc concentration in red blood cells was mostly due to that in CA-I concentration. On the other hand, there were not any changes in total zinc, albumin-bound zinc, and alpha 2-macroglobulin-bound zinc concentrations in plasma during the experimental period. These results, together with the marked hemoconcentration during cold exposure, suggest that cold stress induces an efflux of zinc from plasma.